The production of chlorine by electrolysis of alkali chloride solutions, in particular of sodium chloride brine, is still by far the electrochemical process of highest industrial relevance. As it is well known, different kinds of electrolysis cells are used for this purpose, one of which provides the use of a separator consisting of a semipermeable porous diaphragm, which is nowadays made of a polymer material hydrophilised with inorganic additives.
A description of the functioning of diaphragm chlor-alkali cells is given in Ullmann's Encyclopaedia of Chemical Technology, 5° Ed., Vol. A6, page 424-437, VCH, while an embodiment of cell internal structure is illustrated in the prior art.
Diaphragm cells of the prior art usually comprise rows of intercalated cathodes and anodes, the cathodes being delimited by a conductive surface provided with openings, for instance a mesh or a punched sheet, shaped as a flattened rectangular prism (according to the so-called “cathode finger” geometry) and welded to a peripheral chamber where connections for feeding and discharging the process fluids are arranged. The diaphragm is deposited on the conductive surface of cathodes by vacuum filtering of an aqueous suspension of its constituents. The anodes intercalated to the cathode fingers may be in contact therewith or spaced by a few millimetres. It is, however, necessary to prevent fingers from being subject to flexures in order to avoid damaging the diaphragm by abrasion. Furthermore, during operation the current must be transmitted as uniformly as possible to the whole cathode surface. A non-uniform distribution would lead in fact to a cell voltage increase and to a lessening of the caustic soda generation efficiency, with simultaneous increase of the oxygen content in chlorine. It follows the need of imparting sufficient stiffness and electrical conductivity to the cathodes.
This problem has been addressed, for example, in the prior arty by equipping the cathodes with a longitudinally corrugated carbon steel or copper internal plate. The external conductive surface is secured, preferably by welding, to the apexes of the plate corrugations solving the problems of homogeneous current distribution and of stiffening. Nevertheless, the longitudinal corrugations turn out to be an obstacle to the free motion of hydrogen bubbles, which cannot rise vertically and end up accumulating along the upper generatrix of the fingers, subsequently exiting the peripheral chamber through the relevant outlet. The longitudinally corrugated plate collects hydrogen under each one of the corrugations making it flow therealong longitudinally until discharging through suitable openings in the peripheral chamber: since such flow is difficult to equalise, it follows that the amount of hydrogen present under each corrugation is variable, occluding the facing diaphragm region to a different extent, which leads to a poor current distribution. There has also been described corrugated internal plates, in which corrugations are vertically arranged. Hydrogen can thus be freely collected in the upper part of the fingers, but its flow toward the peripheral chamber is hindered by the upper portion of the corrugations. Moreover, the stiffening effect of vertical corrugations turns out to be unsatisfactory.
More advanced solutions have been proposed in WO 2004/007803 and WO 2006/120002, incorporated herein in their entirety and disclosing the use of plates inserted in the internal volume of the cathode, having discrete protrusions such as bumps, caps or tiles, arranged so as to favour the free circulation of product hydrogen both longitudinally and vertically while attaining an electrical connection with well distributed resistive paths, besides imparting an optimal stiffening to the structure.
The solutions proposed in the prior art are, nevertheless, still unsatisfactory under two standpoints:                under a first aspect, for large-sized cathodes at the most common process current densities (2.5 to 3 kA/m2) the use of internal plates of a highly conductive material such as copper would be preferable in order to improve the current distribution to a sufficient extent. On the other hand, the need to sufficiently stiffen the structure would require copper plates of such a high thickness that this would have a negative impact in terms of costs. It is therefore preferred to manufacture the internal plates out of a material of better mechanical characteristics and/or lower cost, such as carbon steel or different iron or nickel-based materials. The electrical conductivities of steel or nickel are, however, not optimal for big sized cells.        under a second aspect, the internal plate geometries proposed in the cited documents guarantee a good circulation of hydrogen but not a sufficient mixing of the electrolyte inside the cathode. The cathode internal volume is in fact partially occupied by a liquid mixture of process electrolyte and caustic product, whose level normally exceeds half of the cathode height. In such a rather dense phase, concentration and temperature gradients tend to be established, counteracted only in part by natural convection and liable to decrease current efficiency and increase energy consumption and oxygen content in product chlorine.        
It would, therefore, be desirable to have a cathode for electrolysis cells overcoming the limitations of the prior art, particularly as regards current distribution and mixing of the electrolyte inside the internal volume.
In another aspect, it would be desirable to have a diaphragm electrolytic cell overcoming the limitations of the prior art in terms of energy consumption or quality of product chlorine.